Methods and arrangements for device discovery

ABSTRACT

The present disclosure relates to methods and devices for transmission of discovery signal signals and detection of discovery signal signals for device-to-device communication. According to some aspects, the disclosure relates to a method executed in a first wireless terminal for transmitting a control signal for enabling device-to-device, D2D, discovery, wherein the control signal carries an identity. According to one aspect, the method comprises hashing the control signal, taking a time stamp used for the control signal transmission as one input parameter, encoding the hashed control signal and transmitting the encoded signal.

TECHNICAL FIELD

The present disclosure relates to methods and devices for transmissionof discovery signals and detection of discovery signals fordevice-to-device communication.

BACKGROUND

The 3rd Generation Partnership Project, 3GPP, is responsible for thestandardization of the Universal Mobile Telecommunication System, UMTS,and Long Term Evolution, LTE. The 3GPP work on LTE is also referred toas Evolved Universal Terrestrial Access Network, E-UTRAN. LTE is atechnology for realizing high-speed packet-based communication that canreach high data rates both in the downlink and in the uplink, and isthought of as a next generation mobile communication system relative toUMTS. In order to support high data rates, LTE allows for a systembandwidth of 20 MHz, or up to 100 Hz when carrier aggregation isemployed. LTE is also able to operate in different frequency bands andcan operate in at least Frequency Division Duplex, FDD and Time DivisionDuplex, TDD, modes.

Device-to-device communication is a well-known and widely used componentof many existing wireless technologies, including ad hoc and cellularnetworks. Examples include Bluetooth and several variants of the IEEE802.11 standards suite such as Wi-Fi Direct. These systems operate inunlicensed spectrum.

Recently, device-to-device, D2D, communications as an underlay tocellular networks have been proposed as a means to take advantage of theproximity of communicating devices and at the same time to allow devicesto operate in a controlled interference environment. Typically, it issuggested that such device-to-device communication shares the samespectrum as the cellular system, for example by reserving some of thecellular uplink resources for device-to-device purposes. Allocatingdedicated spectrum for device-to-device purposes is a less likelyalternative as spectrum is a scarce resource and dynamic sharing betweenthe device-to-device services and cellular services is more flexible andprovides higher spectrum efficiency.

Devices that want to communicate, or even just discover each other,typically need to transmit various forms of control signaling. Oneexample of such control signaling is the so-called discovery signal,also referred to as beacon signal or discovery beacon signal, which atleast carries some form of identity, referred to as a D2D ID in thisdisclosure. The discovery signal may possibly carry additionalinformation that is useful for the discovery service and is transmittedby a device that wants to be discoverable by other devices.

Other devices may scan for the discovery signal. Once they have detectedthe discovery signal, they can take the appropriate action, for exampleto try to initiate a connection setup with the device transmitting thediscovery signal.

A reference discovery payload of 104 bits of discovery information plus24 bits of CRC may be considered, as an indicative value, according tosimulation assumptions in RAN1. From internal assessments, it resultsthat the D2D ID might be in the order of 80 bits in length.Standardization of D2D is ongoing in 3GPP and more refined numbers arenot available at the moment of writing this disclosure. In thisdisclosure, M refers to the total number of payload bits in a discoverybeacon. From the perspective of this disclosure, it is not essential ifM includes the CRC bits or not, if any.

A user equipment, UE, participating in discovery transmits discoveryinformation that is potentially unique in its discovery signal. A UEthat is trying to discover the first UE will try to extract thediscovery information from the received discovery signal. If the secondUE is successful it will call the first UE as discovered.

ProSe (Proximity Services; see 3GPP feasibility study TR 22.803) definestwo types of discovery: open and restricted. With open discovery, atleast at the first discovery occasion, the discovery information of a UEis not known at receiver in advance. In this case receiving discoveryinformation is simple decoding.

In case of restricted discovery, the receiver attempts detection of acertain specific discovery signal. And the discovery information of thetransmitting UE is known at the receiver before attempting discovery.According to a recent proposal (3GPP contribution paper R1-134627), thereceiver does not need to successfully decode the whole discoveryinformation, at least for restricted discovery. Instead, it may do whatis referred to as “partial bit matching,” which implies that only someof the bits may be correctly decoded by the receiver. An empirical biterror rate (BER) is calculated. If the BER does not exceed a certainthreshold, then the receiver determines that discovery information issuccessfully extracted and the UE is considered as discovered.

Partial bit matching can lead to some false detection. A reason forfalse detection is that some bits may be erroneously decoded which couldlead to an incorrect assumption of a match with the set of N comparedbits. R1-134627 suggests that the false detection probability may becontrolled to some extent by appropriately setting the number of bits tobe correctly matched. On the other hand, increasing N reduces thecomputational efficiency of partial bit matching. Beyond the reducedcomputational complexity associated to correct decoding/detection of Nbits instead of M, R1-134627 states that partial bit matching mayprovide increased detection probability at a given SNR operating pointas compared to the case of full detection. The detection probabilityincreases with a higher BER threshold, however this comes at the cost ofincreased false detection probability.

SUMMARY

An object of the present disclosure is to provide methods andcorresponding devices which seeks to mitigate, alleviate, or eliminateone or more of the above-identified deficiencies in the art anddisadvantages singly or in any combination.

According to some aspects, the disclosure provides a method executed ina first wireless terminal for transmitting a control signal for enablingdevice-to-device, D2D, discovery, wherein the control signal carries anidentity. The method comprises hashing the control signal, taking a timestamp used for the control signal transmission as one input parameter,encoding the hashed control signal and transmitting the encoded signal.

According to some aspects, the control signal comprises signal payloadand wherein the method further comprises inserting the time stamp usedfor control signal transmission in the payload of the control signal,before hashing the control signal.

According to some aspects, the disclosure relates to a first wirelessterminal adapted for transmitting a control signal enablingdevice-to-device, D2D, discovery. The wireless terminal comprisesprocessing circuitry adapted to cause the wireless terminal to hash thecontrol signal, taking a time stamp used for the control signaltransmission as one input parameter, baseband circuitry adapted toencode the hashed control signal and transmit circuitry adapted totransmit the encoded signal.

Some embodiments provide a method executed in a wireless terminal fortransmitting a beacon signal enabling device-to-device discovery. Themethod comprises randomizing the beacon signal by applying arandomization function to at least part of the beacon payload. Forexample, the randomization function may be based on (or, stateddifferently, take as input parameters) one or more of timing information(e.g. the time slot or a time stamp used for beacon transmission),resource index of the beacon transmission, or terminal-specificinformation such as a D2D identity or a portion thereof. In a specificembodiment, the randomization function obtains different results fordifferent beacon transmission occasions. The randomization function maybe e.g. be a hashing function, a polynomial encoder, or a CRC. Thebeacon signal may further be encoded. The wireless terminal thentransmits the beacon signal.

Some embodiments provide a method executed in a wireless terminal fordiscovering another wireless terminal. The wireless terminal isconfigured with a set of one or more identities of wireless terminalswith which device-to-device communication is possible. According to thismethod, the wireless terminal detects the presence of a beacon signal.The wireless terminal descrambles the beacon signal using a descramblingsequence that is based on a selected identity from the set. The wirelessdevice then decodes the beacon signal, and determines if the decodedsignal matches the selected identity. The descrambling and decodingsteps may be repeated until a match is found, or until all identities inthe set have been selected.

Yet further embodiments provide a method executed in a wireless terminalfor discovering another wireless terminal. The wireless terminal isconfigured with a set of one or more identities of wireless terminalswith which device-to-device communication is possible.

According to the method, the wireless terminal detects the presence of abeacon signal and decodes the beacon signal. The wireless terminal thendescrambles the beacon signal using a descrambling sequence that isbased on a selected identity from the set. The wireless terminal thendetermines if the beacon signal matches the selected identity. Thedescrambling step may be repeated until a match is found, or until allidentities in the set have been selected. Stated in a different way, thewireless terminal may perform a de-randomization of the beacon signal,wherein the de-randomization function may be based on correspondingparameters as the randomization function referred to above.

According to another embodiment, a wireless device is provided whichcomprises baseband circuitry, transmit circuitry, a processor and amemory. The wireless terminal is configured with a set of one or moreidentities of wireless terminals with which device-to-devicecommunication is possible. The memory contains instructions executableby said processor, whereby the wireless device is operative to receive abeacon signal, to descramble the beacon signal with a descramblingsequence based on a selected identity from the set, to decode the beaconsignal, and to determine whether the beacon signal matches the selectedidentity. In some variants, the wireless device is a user equipment.

Some embodiments relates to a computer program, comprising computerreadable code which, when run on wireless terminal, causes the wirelessterminal to perform any of the methods described above and below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of the example embodiments, as illustrated in theaccompanying drawings in which like reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe example embodiments.

FIG. 1 illustrates downlink processing of the LTE transport blocks

FIG. 2a illustrates uplink processing of the LTE transport blocks

FIG. 2b shows the proposed interleaving/scrambling procedure(transmitter)

FIG. 2c shows the proposed de-interleaving/descrambling procedure(receiver)

FIG. 3 illustrates an example scenario

FIGS. 4a and 4c illustrates partial bit matching.

FIG. 4b illustrates partial bit matching samples the whole beaconpayload, with hashing.

FIG. 5 illustrates a method in a wireless terminal being a transmitter.

FIGS. 6a and 6b illustrate a method in a wireless terminal being areceiver.

FIG. 7 is a signaling diagram and flowchart combining methods of FIGS. 5and 6.

FIGS. 8a and 8c illustrate an example wireless terminal being atransmitter.

FIGS. 8b and 8d illustrate an example wireless device being a receiver.

FIG. 9 shows a variant of a method in a receiver.

FIG. 10 illustrates a variant of a method in a wireless terminal being atransmitter.

FIGS. 11a and 11b illustrate a variant of a method in a wirelessterminal being a receiver.

FIG. 12 is a signaling diagram and flowchart combining methods of FIGS.10 and 11.

FIG. 13a illustrates partial bit matching being limited to a fraction ofthe beacon payload, without the proposed solution.

FIG. 13b illustrates partial bit matching samples the whole beaconpayload sparsely, with a new interleaver.

FIG. 13c illustrates partial bit matching operates on bits that are afunction of the whole payload, with the new scrambler.

ABBREVIATIONS

CRC cyclic redundancy check

D2D device-to-device

D2D ID device-to-device identity

BER Bit Error Rate

NW Network

DMRS Demodulation Reference Signals

PRB Physical Resource Block

UL uplink

DL downlink

UE user equipment

DETAILED DESCRIPTION

Aspects of the present disclosure will be described more fullyhereinafter with reference to the accompanying drawings. The apparatusand method disclosed herein can, however, be realized in many differentforms and should not be construed as being limited to the aspects setforth herein. Like numbers in the drawings refer to like elementsthroughout.

The terminology used herein is for the purpose of describing particularaspects of the disclosure only, and is not intended to limit theinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise.

Within the context of this disclosure, the terms “wireless terminal” or“wireless device” encompass any terminal which is able to communicatewirelessly with another device, as well as, optionally, with an accessnode of a wireless network, by transmitting and/or receiving wirelesssignals. Thus, the term “wireless terminal” encompasses, but is notlimited to: a user equipment, e.g. an LTE UE, a mobile terminal, astationary or mobile wireless device for machine-to-machinecommunication, an integrated or embedded wireless card, an externallyplugged in wireless card, a dongle etc. Throughout this disclosure, theterm “user equipment” is sometimes used to exemplify variousembodiments. However, this should not be construed as limiting, as theconcepts illustrated herein are equally applicable to other wirelessterminals. Hence, whenever a “user equipment” or “UE” is referred to inthis disclosure, this should be understood as encompassing any wirelessterminal as defined above.

Even though the following examples are described in the context ofdiscovery signals detection for D2D, the principles described in thefollowing disclosure may be equally applied to any detection problemwhere the detected sequence is compared to a known sequence.

As described above, “partial bit matching” may refer to a method whereonly some of the received bits need to be correctly received.

In one example the partial decoder considers the block to be correctlydecoded even if only a subset N out of the number M of bits arecorrectly decoded. This may be the case when the channel is bad e.g. dueto interference, implying that only a part of the payload is correctlydecoded. If the candidate discovery signals differ by more than X bitsit is still possible for the receiver to detect the correct discoverymessage among all candidates even if the received signal has beenpartially corrupted due to noise or interference.

Another example is that the receiver only compares some of the bits outof the decoded information bits, i.e. a subset N out of the number M ofbits, with the known discovery information and count the number of bitsmatched. This also decreases complexity as there are fewer bits to becompared. This solution may be viewed as another type of “partial bitmatching”.

At least two issues are identified with the partial bit matchingproposals mentioned above:

1) If the subsets of N information bits of 2 different discovery signalsare identical, partial bit matching is unable to identify the correctsequence and consistent miss-detection happens.

2) In principle, any two discovery signal payloads may differ by 1-bitonly. This implies that partial bit matching is not applicable inpractice, unless restrictions on the contents of the discovery signalpayloads are enforced. Such restrictions are both inefficient (theywould require redundancy in the definition of the payload lengths) andcomplicated to be enforced in non-coordinated deployments.

The first issues is likely, e.g., if the D2D ID that may occupy ˜80% ofthe discovery signal payload includes area identifiers that are commonto multiple discovery signals, and N<<M. The remaining part of thediscovery signal payload might also include system info that is commonto many UEs.

For D2D discovery, UEs need to be able to discover potentially hundredsor thousands of UEs in proximity. Therefore, implementation efficiencyof the solution is of primary importance.

Some embodiments herein comprise modifying the scrambling and possiblyinterleaving procedure in LTE when transmitting discovery signals forproximity detection, such that discovery signals with a small differencemay be differentiated at the receiver side. This is accomplished byintroducing a new scrambling function in the receiver, also referred toas a randomization function. The purpose is to increase the differencebetween the discovery signals, even when the discovery signal payloadsdiffer by one or a few bits.

The scrambler is split into two parts: a first (new) scrambler operateson the information bits, while the existing scrambler operating on codedbits is modified such that the scrambling sequence is not a function ofUE-specific parameters. Similarly, an interleaver may be added beforethe encoder, leading to certain detection probability advantages.Corresponding steps are performed at the receiver. The benefits of thisapproach are more obvious when it is applied to the partial bitsdetection algorithm that has been proposed. However, effects can also beseen in other cases e.g. when the discovery signal is distorted ornoisy.

In the following part of this disclosure, the term “scrambling” shouldbe interpreted in a wider sense whenever applied to the new scramblerapplied before the channel encoder, as compared to the known XOR-basedbitwise scrambling operation. As stated in some of the embodiments, oneof the justifications for applying scrambling to the discovery signalpayload is that the scrambled version of a same discovery signal shouldbe different at different discovery signal transmissions and consistentcollisions of different discovery signals should be avoided. Anyrandomization function of the discovery signal payload obtaining suchgoal may thus be equivalently applied in this disclosure. Examples ofsuch functions are XOR-based scramblers, CRC, hashing functions,encoders, etc. In some of the embodiments, the scrambling sequence isdefined as a function of parameters that vary at different transmissionoccasions for the same discovery signal payload. E.g., the time slot,any time stamp and/or the resource index used for discovery signaltransmission may be exploited in the generation of the scramblingsequence of the discovery signal. In case the payload randomizationoperation is performed by using a hashing function, a polynomial encoderor a CRC, the time slot and/or any time stamp and/or the resource indexused for discovery signal transmission may be inserted in the payload ofthe discovery signal to be hashed, in order to obtain a randomizationthat is a function of the specific discovery signal transmissioninstance. E.g., the time slot and/or any time stamp and/or the resourceindex used for discovery signal transmission may be prepended orappended to the discovery signal payload before hashing or polynomialencoding or CRC encoding. Any other method for obtaining randomizationof the discovery signal payload to be input to the channel encoder,where the randomization is a function of the discovery signaltransmission instance may be equivalently applied in this disclosure.

Optionally, the randomization may be terminal-specific. Stateddifferently, the randomization may be a function of a terminal-specificparameter, e.g. a D2D identity or a portion thereof. Such a parametermay be applied alone, or in addition to any of the parameters mentionedabove.

Corresponding methods should be applied at the receiver for retrievingat least parts of the original discovery signal payload given theknowledge of the time slot and/or any time stamp and/or the resourceindex parameters used for randomizing discovery signal payload at thetransmitter side.

It is proposed here to address the above issues by a modifiedscrambling/interleaving procedure at the transmitter and receiver of thediscovery signals. As a baseline, the processing of UL and DL LTE datachannels is considered. Even though the procedures described here may beapplied with conventional decoding techniques, one of the benefits ofthe approach described herein is that it can be applied to the partialbits matching technique mentioned above, improving its performance asdescribed later.

For better understanding of the proposed technique, the downlink, DL,and uplink, UL, processing of LTE transport blocks will now be brieflydescribed. FIG. 1 and FIG. 2a show, for reference, the transport block,i.e., information bits, processing in LTE DL and UL. It is noted thatscrambling and interleaving are applied after the encoder, in order toscramble interference in a combination of cell-specific and UE-specificway.

Some embodiments are based on the realization that if the solutions ofFIG. 1 or FIG. 2a are applied to the encoding of discovery signals, thismay alleviate some of the issues listed above. This may be achieved byselecting the scrambling sequences such that they are a function of thepayload of the discovery signal, of timing information (e.g. the timeslot or a time stamp used for beacon transmission) or of resource indexof the beacon transmission.

In one example, this is achieved by scrambling, which may also bereferred to as randomizing, the discovery signal comprising an identity,using a hashing function taking a time stamp used for the discoverysignal transmission as one input parameter. As stated above hashing isconsidered one way of scrambling or randomizing the payload. In thehashing, slight differences in input data may produce very bigdifferences in output data, as will be further described below. When thereceiver is attempting the detection of a specific discovery signal, todetermine if the transmitter is in proximity, it then applies the samehashing that was applied at the transmitter side to the identity that itattempts to detect. Then, the receiver performs decoding and thenperforms bit matching to determine if the received identity matches thehashed identity of the UE that the receiver was attempting to detect. Atthis point a partial bit match may advantageously be performed, becausethe hashing may produce big differences in output data even for signalswith small differences.

In another example, the scrambling sequence for a certain UE may be afunction of the D2D identity, or a portion thereof, thereby renderingthe scrambling sequence UE-specific. When the receiver is attempting thedetection of a specific discovery signal, to determine if thetransmitter is in proximity, it applies the corresponding UE-specificdescrambling code. After descrambling, the receiver performs decodingand then performs bit matching to determine if the received identitymatches the UE the receiver was attempting to detect. At this point apartial bit match may advantageously be performed, as the application ofthe UE-specific descrambling code provides further protection againstfalse detection, even if the number of bits N used for the partial matchis much smaller than M.

FIG. 3 illustrates an example scenario in which various embodimentsdescribed herein may operate. This example shows a number of wirelessterminals 10, 20, 30 and 40 that are capable of D2D communication. Oneor more of the wireless terminals may, for example, be LTE userequipments. In this example, terminals 30 and 40 have alreadyestablished D2D communication which each other. Wireless terminal 10 istransmitting a discovery signal 12 (shown as a shaded circle surroundingterminal 10), which comprises an identity of the wireless terminal 10.Wireless terminal 20 is in close enough proximity to terminal 10 to beable to detect the discovery signal. Of course, other wireless terminalsmay also be present and some of these terminals may also be transmittingdiscovery signal signals, and/or be in D2D communication with otherterminals. It should further be noted that wireless terminals 10 and 20may be comprised in a wireless network 100, e.g. a cellular network. Thewireless network may for example be an LTE or UMTS network. In thatcase, one or more of wireless terminals 10 and/or 20 may be connected tothe wireless network.

The claimed method proposes splitting the scrambling and/or interleavingblock into two blocks. The first block is applied to the informationbits and the second block after the encoder (as in 3GPP TS 36.211V12.3.0—FIG. 5-3.1), shown in FIG. 2b . Corresponding descramblingand/or de-interleaving steps are performed at the receiver, shown inFIG. 2 c.

The interleaving step applied after the encoder may be a function of thecell-ID, the time slot, the resource index, or any other non UE-specificparameters (differently from LTE). Possibly, the interleaver may bestatic and pre-defined.

Several embodiments herein are thus based on the understanding thatdecoding is more computationally demanding than descrambling and/orde-interleaving. These embodiments allow significant computationaladvantage at the receiver. Additionally, these embodiments allow solvingor at least alleviating the issues with the partial bits matchingproposal. From a computational perspective, these embodiments requireonly one decoder iteration.

With reference to the scenario in FIG. 3 and the flowchart in FIG. 5, amethod performed in a first wireless terminal 10, e.g. an LTE userequipment, for transmitting a control signal for enablingdevice-to-device, D2D, discovery, wherein the control signal carries anidentity, wherein scrambling is performed before encoding, will now bedescribed. In FIG. 5 the discovery signal is randomized using a hashingfunction.

It should be appreciated that FIG. 5 comprises some operations which areillustrated with a darker border and some operations which areillustrated with a lighter border. The operations which are comprised ina darker border are operations which are comprised in the broaderexample embodiment. The operations which are comprised in a lighterborder are example embodiments which may be comprised in, or a part of,or are further operations which may be taken in addition to theoperations of the border example embodiments. It should be appreciatedthat these operations need not be performed in order. Furthermore, itshould be appreciated that not all of the operations need to beperformed. The example operations may be performed in any order and inany combination.

Both interleaver and scrambling blocks may be introduced at thetransmitter, before encoding. However, the disclosed concept covers alsothe case where only the scrambler and only the interleaver is inserted.Such blocks provide different and combinable benefits, as explainedherein. Obviously, only the corresponding blocks should be introduced atthe receiver. The example in FIG. 5 comprises only a scrambler. In FIG.5 the scrambler is implemented as a hashing function.

The scrambling step applied before the encoder is initialized by aninput or scrambling sequence that is a function of the discovery signalpayload and possibly additional parameters that may vary at successivediscovery signal transmissions. E.g., the time slot and the resourceindex for discovery signal transmission may be examples of suchparameters. The rationale is that the scrambled version of a certaindiscovery signal should be different at different discovery signaltransmissions and consistent collisions of different discovery signalsshould be avoided.

The interleaving step applied after the encoder may be a function of thecell-ID, the time slot, the resource index, or any other non UE-specificparameters (differently from LTE). Possibly, the interleaver may bestatic and pre-defined.

The first wireless terminal 10 is configured to transmit a discoverysignal, here referred to as a control signal, comprising an identityassociated with the terminal 10. This enables other wireless terminal,such as terminal 20, to discover the presence of terminal 10 andinitiate device-to-device communications with it.

According to the proposed method, the first wireless terminal 10scrambles S1 the control signal before the encoding, with a scramblingsequence which is based on, or dependent on, the identity associatedwith the terminal 10. Stated differently, the scrambling sequence is afunction of the identity of the wireless terminal. Stated in yet anotherway, the scrambling sequence is a function of at least oneuser-specific, or terminal-specific, input parameter.

In some embodiments, the scrambling implies that the wireless devicehashes S1 the control signal, taking a time stamp used for the controlsignal transmission as one input parameter. The time stamp is e.g. adate-time format and shall be encoded according to Annex B.2.1.2 of 3GPPTS 33.220 V12.2.0. The control signal comprises signal payload andaccording to some aspects, the method further comprises inserting S0 thetime stamp used for control signal transmission in the payload of thecontrol signal, before hashing the control signal. The hashing functionuses a hashing key that is e.g. specific to a restricted users group.

FIGS. 4a and 4b illustrates that partial bit matching is unable todifferentiate signals wherein the payload only differs in one of a fewbits. In FIG. 4a all bits are not matched. In FIG. 4b three bits arewrongly decoded, which leads to that the two packets cannot bedifferentiated.

FIG. 4b illustrates that partial bit matching can differentiate thediscovery signals of FIG. 4a after applying a hashing function on thediscovery signals, taking a time stamp used for the control signaltransmission as one input parameter. In the hashing, slight differencesin input data may produce big differences in output data.

Alternatively, the input parameter to the scrambling may be based on, ordependent on, some part of portion of the identity. In a particularvariant, the identity comprises one portion which is shared by severalwireless terminals (e.g. comprising an area identifier or other commoninformation), and another portion which is unique to the wirelessterminal 10, and the scrambling sequence is based on, or dependent on,the portion of the identity which is unique to the wireless terminal 10.

In a further alternative, the input parameter e.g. the scramblingsequence is a function of all of, or a part of, the payload of thediscovery signal.

Optionally, the wireless terminal 10 also performs interleaving, notshown, of the discovery signal. If interleaving is applied, it may beperformed before and/or after the encoding step. When interleaving isperformed before encoding, as shown in FIG. 13, it may be based eitheron non-terminal specific parameters (also referred to as commonparameters), or on terminal-specific parameters, or a combination ofboth. However, when interleaving is performed after encoding, theinterleaving should preferably not be based on terminal-specificparameters, as the receiver would then be required to perform thedecoding step multiple times.

According to the method, wireless terminal 10 then encodes S2 thediscovery signal, i.e. the hashed control signal. Hence, in thisembodiment encoding is performed after performing terminal-specificscrambling. A benefit of this approach is that it enables an efficientdetection of the discovery signal at the receiver side, as the receiverwill not be required to decode the signal multiple times.

The wireless terminal 10 then transmits S4 the discovery signal,enabling discovery by other devices and potentially also initiation ofD2D communication with such devices.

In some variants, an additional scrambling step S3 may optionally beperformed after encoding (and before transmission). The discovery signalis then scrambled with a second scrambling sequence which is notdependent on any terminal-specific parameters, or stated differently,not based on or dependent on the terminal's identity, or stated in yetanother way, not based on or dependent on the discovery signal payload.The second scrambling sequence may instead be based on commonparameters, e.g. the time slot and/or resource index of the discoverysignal transmission.

In yet further variants, interleaving may be performed before as well asafter encoding. In this case, the interleaving step performed afterencoding is preferably based only on non-terminal-specific parameters,in order to maintain the benefit of more efficient detection on thereceiver side. The interleaving step performed before encoding may bebased on terminal-specific parameters and potentially also on commonparameters.

The corresponding detection procedure is exemplified in 6 a. Thereceiver performs descrambling/de-interleaving according to thenon-beacon-specific scrambling sequence/interleave initialization. Theresulting sequence is decoded only once. Once detection is performed,the receiver iterates multiple detection attempts using the list of UEspotentially in proximity.

In one example, at each detection attempt the decoded bits are comparedto the interleaved and/or scrambled hypothesis according to theinterleaver and/or scrambler defined in this disclosure.

In another implementation alternative, at each detection attempt thedecoded bits are de-interleaved and descrambled according to theinterleaver and/or scrambler defined in this disclosure and compared tothe payload of the hypothesis beacon.

Combinations of the above examples are possible. e.g., if thede-interleaver is not UE-specific, it may be applied directly after thedecoder avoiding to iterate it.

In one example, one detection attempt may be performed for each of theUEs potentially in proximity.

In another example, the detection attempts are stopped as soon as onedetection attempt based on one of the UEs potentially in proximity issuccessful.

In a further example, the detection step may be performed according tothe partial bits matching technique. In this case, only N out of M bitsare decoded and compared to the expected bits. A non-zero BER thresholdmay be considered for declaring the beacon detection successful. Thecomparison may be equivalently performed on the scrambled or interleavedinformation bits.

With reference to the scenario in FIG. 3 and the flowchart in FIG. 6a ,a method performed in a second wireless terminal 20, e.g. an LTE userequipment, for discovering a first wireless terminal 10, wherein thesecond wireless terminal is configured with a set of one or moreidentities of wireless terminals with which device-to-device, D2D,communication is possible, will now be described. This is thereceiver-side method corresponding to the method shown in FIG. 5.

The wireless terminal 20 receives S11, or detects the presence of, acontrol signal. The control signal is a discovery signal transmittedfrom terminal 10. This discovery signal comprises an identity associatedwith the terminal 10. The wireless terminal is configured with a list(or a set) of one or more identities of wireless terminals for which D2Dcommunication is possible. This list may, for example, be configured bywireless network 100 or by some other means. According to some aspects,the identity comprises an identity of the wireless terminal 10.According to some aspects, the identity comprises a ProSe user identityand/or an application layer user identity.

Optionally, the wireless terminal descrambles S12 the signal using anon-terminal-specific descrambling sequence. This option applies if acorresponding non-terminal-specific scrambling step was performed by thetransmitter.

The wireless terminal then decodes S13 the signal. The decoding shouldof course be performed such that the encoding that was performed by thetransmitter is reversed. The decoding parameters to use, may e.g. bepreconfigured by reference to a technical standard, or may be configuredvia signaling e.g. from network 100.

Optionally, de-interleaving of the signal is performed, if acorresponding interleaving step was performed by the transmitter.

In the receiver, the identity which the receiver is attempting to matchis scrambled S14 (potentially together with additional informationcomprised in the discovery signal payload), and optionally interleavedif interleaving was applied by the transmitter. In this example thescrambling is implemented as a hashing function.

Hence, in the example embodiment using hashing, this step implieshashing S14 a reference control signal comprising one of the one or moreidentities, taking a time stamp used for the control signal transmissionas one input parameter. Then, the received discovery signal is comparedS15 with the scrambled (and optionally interleaved) information in orderto determine if there is a match. Steps S14 and S15 are performed foreach identity until all the identities in the set have been compared oruntil a match is found.

In other words, detection may be achieved either bydescrambling/de-interleaving the received information, or conversely byscrambling/interleaving the data that the received information is to becompared against.

FIG. 7 illustrates how the transmitter-side method of FIG. 6 mayinteract with the receiver side method of FIG. 5.

An advantage of applying a user-specific, or terminal-specific,scrambling sequence, as shown in FIGS. 5-7, is that when partial bitmatching is applied, the risk of false detection as described above isminimized or at least reduced.

Variations of the claimed technique will now be described referring toFIGS. 9-12.

In one implementation alternative shown in FIG. 9, at each detection S11attempt the decoded bits are de-interleaved and descrambled S12according to the interleaver and/or scrambler defined in this disclosureand compared to the payload of the hypothesis beacon.

As in the example of FIG. 6b , the wireless terminal 20 then decodes S13the signal. The decoding should of course be performed such that theencoding that was performed by the transmitter is reversed. Whichdecoding parameters to use may e.g. be preconfigured by reference to atechnical standard, or may be configured via signaling e.g. from thewireless network 100.

Optionally, de-interleaving of the signal is performed, if acorresponding interleaving step S12 was performed by the transmitter.

In order to determine if the detected beacon corresponds to one of theknown wireless terminals for which D2D communication is enabled, thewireless terminal 20 repeatedly attempts to descramble S14 b and matchS15 b the received beacon signal with known identities from the list,until a match is found or until there are no more known identities totry. It is also possible for the terminal to keep testing for knownidentities even after a match is found, in order to further reduce therisk of false detection. Notably, in this embodiment the decoding stepdoes not need to be repeated, because the transmitter performedterminal-specific scrambling before the encoding step (see FIG. 5). Ifterminal-specific interleaving was performed on the transmitter side, aswill be described in FIG. 10, the de-interleaving step will have to berepeated for each identity which is to be tested.

The wireless terminal 20 descrambles the signal using aterminal-specific descrambling sequence which is based on, or dependenton, one of the known identities in its list. The descrambling sequenceshould of course be selected in order to undo the scrambling that wasperformed on the transmitter side, hence all the alternatives describedin connection with FIG. 5 for obtaining the scrambling sequence applymutatis mutandis.

Knowledge of how to select the descrambling sequence may for example beencoded in a technical standard, or possibly signaled to the wirelessterminal 20 e.g. from the wireless network 100.

The wireless terminal 20 then determines if the beacon signal, or morespecifically the payload of the beacon corresponding to the identity ofthe wireless terminal 20, matches the known identity (the same identitythat was applied in the descrambling step). This may be performed bydoing a complete bit match, or advantageously by partial bit matching ofa subset N of the M bits that make up the identity, as has beendescribed above.

Assuming that the list of known terminals includes the identity ofwireless terminal 10, i.e. the method of FIG. 15 results in a match, thewireless terminal 20 may then proceed to initiate or setup D2Dcommunications with the wireless terminal 10.

It is pointed out that although our example suggested scrambling beingperformed before interleaving, the order of these steps may be reversedin some embodiments. The order of the descrambling and de-interleavingsteps at the receiver side will then be modified accordingly so thatdescrambling is performed before the de-interleaving step. However, thisalso implies that both descrambling and de-interleaving need to bereiterated for each identity that the receiver attempts to match.

Another example method executed in a wireless terminal 10, e.g. an LTEuser equipment, will now be described with reference to FIG. 3 and theflowchart shown in FIG. 10. In this example the new scrambler S3 b(which corresponds to both steps S1 and S3 in FIG. 5) is implementedafter the encoder S2 b.

The wireless terminal 10 is configured to transmit a discovery signalcomprising an identity associated with the wireless terminal 10. Thisenables other wireless terminal, such as the wireless terminal 20, todiscover the presence of the wireless terminal 10 and initiatedevice-to-device communications with it.

According to the method, the wireless terminal 10 encodes S2 thediscovery signal. The wireless terminal 10 then scrambles S3 b thesignal with a scrambling sequence which is based on, or dependent on,the identity associated with the wireless terminal 10. Stateddifferently, the scrambling sequence is a function of the identity ofthe wireless terminal 10. Stated in yet another way, the scramblingsequence is a function of at least one user-specific, orterminal-specific, input parameter.

Alternatively, the scrambling sequence may be based on, or dependent on,some part of portion of the identity. In a particular variant, theidentity comprises one portion which is shared by several wirelessterminals (e.g. comprising an area identifier or other commoninformation), and another portion which is unique to the wirelessterminal 10, and the scrambling sequence is based on, or dependent on,the portion of the identity which is unique to the wireless terminal 10.

In a further alternative, the scrambling sequence is a function of allof, or a part of, the payload of the beacon.

Optionally, the wireless terminal 10 also performs interleaving S35 b ofthe beacon signal.

The wireless terminal 10 then transmits the beacon signal, enablingdiscovery by other devices.

A corresponding method performed in a wireless terminal on the receivingside, e.g. wireless terminal 20 in FIG. 3, will now be described withreference to the flow chart in FIG. 11 b.

The wireless terminal receives, or detects S11 c the presence of, abeacon signal from wireless terminal 10. The wireless terminal isconfigured with a list of one or more identities of wireless terminalswith which D2D communication is possible (or, stated differently,wireless terminals with which the wireless terminal 10 is allowed to, orenabled to, establish D2D communication). This list may, for example, beconfigured by the wireless network 100 or by some other means.

Optionally, the wireless terminal de-interleaves S115 c the signal. Thisoption applies if a corresponding interleaving step S35 was performed atthe transmitter (see FIG. 10).

In order to determine if the detected beacon corresponds to one of theknown wireless terminals for which D2D communication is enabled, thewireless terminal repeatedly attempts to decode S13 c and match S15 cthe received beacon signal with known identities from the list, until amatch is found or there are no more known identities to try. It is alsopossible for the terminal to keep testing for known identities evenafter a match is found, in order to further reduce the risk of falsedetection.

The wireless terminal 20 descrambles S14 c the signal using adescrambling sequence which is based on, or dependent on, one of theknown identities in its list. The descrambling sequence should of coursebe selected in order to undo the scrambling that was performed on thetransmitter side, hence the same alternatives as described inconjunction with FIG. 10 apply.

Knowledge of how to select the descrambling sequence may for example beencoded in a technical standard, or possibly signaled to the wirelessterminal 20 e.g. from the wireless network 100.

The wireless terminal 20 then attempts to decode the signal. Once againthe decoding should of course be performed such that the encoding thatwas performed by the transmitter is reversed.

The wireless terminal 20 then determines if the beacon signal, or morespecifically the payload of the beacon corresponding to the identity ofthe wireless terminal, matches the known identity (the same identitythat was applied in the descrambling step). This may be performed bydoing a complete bit match, or advantageously by partial bit matching ofa subset N of the M bits that make up the identity, as has beendescribed above.

Assuming that the list of known terminals includes the identity of thewireless terminal 10, i.e. the method of FIG. 12 results in a match, thewireless terminal 20 may then proceed to initiate or setup D2Dcommunications with the wireless terminal 10.

FIG. 12 is a combined signaling diagram and flow chart which shows howthe methods of FIG. 10 (transmitter) and FIG. 11b (receiver) mayinteract.

The methods of FIG. 10-12 may be applied in particular in a scenariowhere the possible number of other devices, e.g. UE:s, that may bedetected by a receiver is limited—or stated differently, when the listof known identities that the wireless terminal 20 is configured toattempt to decode is reasonably short.

However, a possible drawback of this solution is that the receiver needsto perform a decoding attempt for each UE potentially in proximity.Assuming that the receiver may have at any time a list of hundreds orthousands of UEs potentially in proximity, the computational complexityinduced by so many decoding attempts might be excessive. The detectionprocedure is exemplified in FIG. 11 a.

FIG. 8a illustrates an example wireless terminal in which any of themethods of FIG. 2b , 5 or 10 may be implemented.

According to an embodiment, a wireless device 10 is provided whichcomprises means adapted to scramble a discovery signal with aterminal-specific scrambling sequence, to encode the discovery signal,and to transmit the discovery signal. In some variants, to scrambleimplies to hash the control signal, taking a time stamp used for thecontrol signal transmission as one input parameter. In some variants,the wireless device is a user equipment. The wireless device may furtherbe adapted to perform any of the method steps described in connectionwith FIG. 2b , 5 or 10.

In a particular embodiment, illustrated in FIG. 8a , the wireless devicemay comprise baseband circuitry 103 adapted to scramble the discoverysignal with a terminal-specific scrambling sequence, and to encode thediscovery signal. The wireless device may further comprise transmitcircuitry 102, typically a radio, Rf, transmitter, adapted to transmitthe discovery signal. The transmit circuitry may be associated with oneor more physical antennas over which the discovery signal istransmitted. In some variants the wireless device may further comprise aprocessor 101 and a memory 104. In some variants, the processor isadapted to hash the control signal, taking a time stamp used for thecontrol signal transmission as one input parameter.

In another particular embodiment, illustrated in FIG. 8c , the wirelessdevice 10 may comprise a scrambler adapted to scramble the discoverysignal with a terminal-specific scrambling sequence, and an encoderadapted to encode the discovery signal. In some variants, the scrambleris adapted to hash the control signal, taking a time stamp used for thecontrol signal transmission as one input parameter. The wireless devicemay optionally comprise an interleaver adapted to interleave thediscovery signal. The scrambler, encoder and interleaver may be providedin a baseband processing unit 103. The baseband processing unit may bethe baseband circuitry 103 shown in FIG. 8 a.

The wireless device may further comprise a transmitting unit adapted totransmit the discovery signal, e.g. via the transmit circuitry andantenna shown in FIG. 8 a.

According to another embodiment, a computer program is providedcomprising instructions which, when executed on at least one processor(e.g. the processor shown in FIG. 8a ), cause the at least one processorto carry out the method described in connection with any one of FiguresFIG. 2b , 5 or 10.

According to yet another embodiment, a carrier is provided containingthe computer program described in the previous paragraph. The carriermay be an electronic signal, optical signal, radio signal, or computerreadable storage medium.

FIG. 8b illustrates an example wireless device 20 in which any of themethods of FIG. 2c , 6, 9 or 11 may be implemented. The wireless device20 is adapted for discovering first wireless terminal, wherein thesecond wireless terminal is adapted with a set of one or more identitiesof wireless terminals with which device-to-device, D2D, communication ispossible

According to an embodiment, a wireless device is provided whichcomprises means adapted to store a set of one or more identities ofwireless terminals with which device-to-device communication ispossible, to receive a discovery signal, to descramble the receiveddiscovery signal with a descrambling sequence which is based on aselected identity from the set, to decode the discovery signal, and todetermine if the decoded signal matches the selected identity.Alternatively, a wireless device is provided which comprises meansadapted to store a set of one or more identities of wireless terminalswith which device-to-device communication is possible, to receive adiscovery signal, to decode the discovery signal, to scramble a selectedidentity from the set, and to determine if the decoded signal matchesthe scrambled selected identity. According to some aspects to descramblea selected identity implies to hash a reference control signalcomprising one of the one or more identities, taking a time stamp usedfor the control signal transmission as one input parameter.

In some variants, the wireless device is a user equipment. The wirelessdevice may further be adapted to perform any of the method stepsdescribed in connection with FIG. 2c , 6, 9 or 11.

In a particular embodiment, illustrated in FIG. 8b , the wireless devicemay comprise receive circuitry adapted to receive the discovery signal.The wireless device may further comprise baseband circuitry 203 adaptedto scramble an identity (or to descramble the discovery signal with thedescrambling sequence), and to decode the discovery signal.Alternatively the descrambling or scrambling is performed in aprocessing circuitry 201. According to some aspects the basebandcircuitry 203 or processing circuitry 201 is adapted to hash a referencecontrol signal comprising one of the one or more identities, taking atime stamp used for the control signal transmission as one inputparameter and to compare the output signal of the hashing with thedecoded signal, for each identity until all the identities in the sethave been selected or until a match is found.

The receive circuitry 202 may be associated with one or more physicalantennas over which the discovery signal is received. The wirelessdevice may further comprise a memory 204 adapted to store a set of oneor more identities of wireless terminals with which device-to-devicecommunication is possible. In some variants the wireless device mayfurther comprise a processor.

In another particular embodiment, illustrated in FIG. 8d , the wirelessdevice may comprise a descrambler adapted to descramble the discoverysignal (or to scramble a reference control signal comprising one of theone or more identities), and a decoder adapted to decode the discoverysignal. Further, the wireless device 20 may comprise a receiving unitadapted to receive the discovery signal. The wireless device may alsocomprise means for storing the set of one or more identities in astorage unit. The wireless device may optionally comprise ade-interleaver adapted to de-interleave the discovery signal. Thedescrambler, decoder and de-interleaver may be provided in a basebandprocessing unit 203. The baseband processing unit 203 may be thebaseband circuitry 203 shown in FIG. 8b . The storage unit may be thememory 204 shown in FIG. 8 b.

According to another embodiment, a computer program is providedcomprising instructions which, when executed on at least one processor(e.g. the processor shown in FIG. 8b ), cause the at least one processorto carry out the method described in connection with any one of FIG. 2c, 6, 9 or 11.

According to yet another embodiment, a carrier is provided containingthe computer program described in the previous paragraph. The carriermay be an electronic signal, optical signal, radio signal, or computerreadable storage medium.

In light of the embodiments presented above, the issues associated topartial bits matching may be solved or at least alleviated wheninterleaving and/or scrambling is performed before encoding. In moredetail, advantages of such methods include. FIG. 13a illustrates thatwithout the proposed solution, partial bit matching is limited to afraction of the beacon payload.

If an interleaver is provided before the encoder (and correspondingde-interleaver at the receiver) according to embodiments herein, theinformation bits are spread pseudo-randomly such that different portionsof the information payload are sampled in the N bits selected forpartial bits matching, reducing the probability of false detection.E.g., the D2D ID may comprise a geographical area-identifier part thatis common to multiple discovery signals. If the N bits happen to beextracted from such field, the detection would be ambiguous. On theother hand, the interleaver proposed here ensures that the fields withinthe discovery signal are sampled pseudo-randomly FIG. 13b illustratesthat with the proposed interleaver, partial bit matching samples thewhole beacon payload sparsely.

If a scrambler is introduced before the encoder (and a correspondingdescrambler at the receiver) according to embodiments herein, theprobability of consistent discovery signal false detection may beminimized or at least reduced. It is assumed, in some embodiments, thatthe scrambling sequence is a combination of the discovery signal payloadand other parameters (e.g., time slot index, resource index, etc.) thatvary at different periodic transmissions of the same discovery signal.Therefore, the probability that N scrambled bits of two differentdiscovery signals match at consecutive discovery signal receptions isminimized. FIG. 13c illustrates that with the proposed scrambler,partial bit matching operates on bits that are a function of the wholepayload.

If the scrambling sequence is a function of the whole discovery signalpayload, or at least of the portion of the payload that corresponds tothe D2D identity (or a significant fraction thereof), the advantages ofthe interleaver solution may be obtained by introducing only thescrambler.

Aspects of the disclosure are described with reference to the drawings,e.g., block diagrams and/or flowcharts. It is understood that severalentities in the drawings, e.g., blocks of the block diagrams, and alsocombinations of entities in the drawings, can be implemented by computerprogram instructions, which instructions can be stored in acomputer-readable memory, and also loaded onto a computer or otherprogrammable data processing apparatus. Such computer programinstructions can be provided to a processor of a general purposecomputer, a special purpose computer and/or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer and/or otherprogrammable data processing apparatus, create means for implementingthe functions/acts specified in the block diagrams and/or flowchartblock or blocks.

In some implementations and according to some aspects of the disclosure,the functions or steps noted in the blocks can occur out of the ordernoted in the operational illustrations. For example, two blocks shown insuccession can in fact be executed substantially concurrently or theblocks can sometimes be executed in the reverse order, depending uponthe functionality/acts involved. Also, the functions or steps noted inthe blocks can according to some aspects of the disclosure be executedcontinuously in a loop.

In the drawings and specification, there have been disclosed exemplaryaspects of the disclosure. However, many variations and modificationscan be made to these aspects without substantially departing from theprinciples of the present disclosure. Thus, the disclosure should beregarded as illustrative rather than restrictive, and not as beinglimited to the particular aspects discussed above. Accordingly, althoughspecific terms are employed, they are used in a generic and descriptivesense only and not for purposes of limitation.

It should be noted that although terminology from 3GPP LTE has been usedherein to explain the example embodiments, this should not be seen aslimiting the scope of the example embodiments to only the aforementionedsystem. Other wireless systems, including WCDMA, WiMax, UMB and GSM, mayalso benefit from the example embodiments disclosed herein.

The description of the example embodiments provided herein have beenpresented for purposes of illustration. The description is not intendedto be exhaustive or to limit example embodiments to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of various alternativesto the provided embodiments. The examples discussed herein were chosenand described in order to explain the principles and the nature ofvarious example embodiments and its practical application to enable oneskilled in the art to utilize the example embodiments in various mannersand with various modifications as are suited to the particular usecontemplated. The features of the embodiments described herein may becombined in all possible combinations of methods, apparatus, modules,systems, and computer program products. It should be appreciated thatthe example embodiments presented herein may be practiced in anycombination with each other.

It should be noted that the word “comprising” does not necessarilyexclude the presence of other elements or steps than those listed andthe words “a” or “an” preceding an element do not exclude the presenceof a plurality of such elements. It should further be noted that anyreference signs do not limit the scope of the claims, that the exampleembodiments may be implemented at least in part by means of bothhardware and software, and that several “means”, “units” or “devices”may be represented by the same item of hardware.

Although the description is mainly given for a user equipment, asmeasuring or recording unit, it should be understood by the skilled inthe art that “user equipment” is a non-limiting term which means anywireless device, terminal, or node capable of receiving in DL andtransmitting in UL (e.g. PDA, laptop, mobile, sensor, fixed relay,mobile relay or even a radio base station, e.g. femto base station).

The various example embodiments described herein are described in thegeneral context of method steps or processes, which may be implementedin one aspect by a computer program product, embodied in acomputer-readable medium, including computer-executable instructions,such as program code, executed by computers in networked environments. Acomputer-readable medium may include removable and non-removable storagedevices including, but not limited to, Read Only Memory (ROM), RandomAccess Memory (RAM), compact discs (CDs), digital versatile discs (DVD),etc. Generally, program modules may include routines, programs, objects,components, data structures, etc. that perform particular tasks orimplement particular abstract data types. Computer-executableinstructions, associated data structures, and program modules representexamples of program code for executing steps of the methods disclosedherein. The particular sequence of such executable instructions orassociated data structures represents examples of corresponding acts forimplementing the functions described in such steps or processes.

In the drawings and specification, there have been disclosed exemplaryembodiments. However, many variations and modifications can be made tothese embodiments. Accordingly, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the embodiments being defined bythe following claims.

The invention claimed is:
 1. A method executed in a first wirelessterminal for transmitting a control signal enabling device-to-device(D2D) discovery, wherein the control signal carries an identity, themethod comprising: hashing the control signal, taking a time stamp usedfor the control signal transmission as one input parameter; encoding thehashed control signal; and transmitting the encoded signal.
 2. Themethod of claim 1, wherein the identity comprises an identity of thefirst wireless terminal.
 3. The method of claim 1, wherein the identitycomprises one portion that is shared by several wireless terminals. 4.The method of claim 1, wherein the control signal comprises signalpayload and wherein the method further comprises inserting the timestamp used for control signal transmission in the payload of the controlsignal, before hashing the control signal.
 5. The method of claim 1,wherein the method further comprises scrambling the encoded signal.
 6. Afirst wireless terminal adapted for transmitting a control signalenabling device-to-device (D2D) discovery, wherein the control signalcarries an identity, the first wireless terminal comprising: processingcircuitry adapted to cause the wireless terminal to hash the controlsignal, taking a time stamp used for the control signal transmission asone input parameter; baseband circuitry adapted to encode the hashedcontrol signal; and transmit circuitry adapted to transmit the encodedsignal.
 7. The first wireless terminal of claim 6, wherein the identitycomprises an identity of the first wireless terminal.
 8. The firstwireless terminal of claim 6, wherein the identity comprises a ProSeuser identity or an application layer user identity, or both.
 9. Thefirst wireless terminal of claim 6, wherein the control signal comprisessignal payload and wherein the processing circuitry is further adaptedto insert the time stamp used for control signal transmission in thepayload of the control signal, before hashing the control signal.
 10. Awireless terminal of claim 6 wherein the first wireless terminal is auser equipment.
 11. A non-transitory computer-readable mediumcomprising, stored thereupon, a computer program comprising computerreadable code that, when run on a wireless terminal, causes the wirelessterminal to: hash a control signal enabling device-to-device (D2D)discovery, wherein the control signal carries an identity, taking a timestamp used for the control signal transmission as one input parameter tothe hashing of the control signal; encode the hashed control signal; andtransmit the encoded signal.